The present invention relates to an improved apparatus which is suitable in particular for the manufacture of pipes made of plastic which is molecularly oriented by radial expansion by means of a fluid, from an unoriented plastic pipe section which is at a temperature at which drawing induces a molecular orientation.
A process and an apparatus which are suitable in particular for producing molecularly oriented plastic pipes according to this drawing technique are already the subject of Patent Application No. FR-A-92,510,940 in the name of SOLVAY & Co.
According to this process, a pipe section is placed in a mould, radial expansion is produced of the pipe section which is heated to a temperature at which drawing induces molecular orientation of the plastic by means of a fluid which is introduced under pressure into the pipe section after the latter has been closed at both of its ends, the resulting pipe is cooled and extracted from the mould, the said process being characterised in that the pipe section is clamped in a sleeve and a relative displacement of the sleeve along the pipe section is produced while fluid is being introduced therein, so as to cause radial expansion of the pipe section downstream of the sleeve. In this process, the start of the radial expansion of the pipe section to be oriented always takes place at one end and the expansion bubble formed extends gradually along the pipe section, this progression being controlled by the relative displacement of the sleeve. This process makes it possible to produce oriented pipes the radial orientation of which is perfectly controlled even for pipes of great length.
The apparatus which can be employed to implement this process incorporates a tubular mould the transverse dimensions of which correspond to those of the pipe to be produced and which is fitted with a device for admitting a pressurized fluid into a zone of the mould which is intended to receive the pipe section to be expanded radially, a member for closing and grasping one end of the pipe section in the mould, a tubular sleeve which opens into the mould by an open end facing the closing and grasping member and which has transverse dimensions corresponding to those of the pipe section which it is intended to receive, and means for causing a controlled relative axial displacement of the sleeve in relation to the closing and grasping member, for moving them apart and for extracting the pipe section from the sleeve and which is characterised in that the open end of the sleeve is fitted with an annular plunger mounted so as to slide in the mould.
According to the abovementioned patent application, the annular plunger has an annular surface arranged at the exit of the sleeve and extending towards the inner wall of the mould, of a generally frustoconical shape and widened out in the direction of the closing and grasping member, the conicity angle being between 15.degree. and 75.degree..
It has been found, however, that when such a plunger is used in the apparatus described above the degree of longitudinal orientation of the molecularly oriented pipes which are produced is not sufficiently reproducible and controllable. Moreover, it has been found that the molecularly oriented pipes produced can exhibit irregularities in wall thickness.